1. Field of the Invention
This invention relates to a positive displacement pump which operates by a combination of orbital and nodding movements, and is capable of continuous adjustment from zero to maximum flow.
2. Description of Related Art
Operations in which pumps must function reliably and satisfy special requirements range widely. In pumping human blood, for instance into and through dialysis machines, the fluid to be moved is extremely delicate, and must not be roughly handled or subjected to steep pressure gradients or high shears and, of course, must be kept scrupulously clean. In contrast are pumps for sewage; such pumps must have long life, reasonable cost, and tolerance of anything that can appear in a sewage system, including metals, flexible solids, corrosives and abrasives and fibrous materials. For blood, one form of pump used is a series of rollers mounted on a wheel, bearing against a constrained, flexible plastic tube, a variety of the peristaltic type. In contrast, sewage pumps are usually of either the progressing cavity or velocity head (centrifugal) type, which sliding shoe pumps are used to move suspended abrasives, such as mine de-watering operations or sludge pumping.
The nature of the fluid to be pumped--particularly its viscosity--plays a considerable part in the selection of the type of mechanism to be adopted, as well as the materials to be used. The fluid may be volatile, corrosive, abrasive, viscous and/or delicate; it may also be non-homogeneous and possess more or less lubricity.
Despite the huge variety of uses of pumps, there are only two main types--velocity head and positive displacement. The velocity head type, most of which are centrifugal pumps, are usually used to deliver low viscosity fluids at relatively low pressures; although the feedwater pumps used in power stations are multi-stage centrifugal machines which deliver up to 3,500 psi at large flow rates.
Velocity-head pumps develop their performance by generating high fluid velocity; this is converted to pressure in specially shaped divergent passages. To accommodate high fluid velocities, specially shaped passages in both rotating and static parts are used, and these passages may be costly to produce. But while ordinarily not mechanically complex, velocity heads are usually unsuitable for use with high viscosity fluids (because of shear resistance) or with suspended matter unless modified by rubber lining or special wear-resistant casings.
While delivery of velocity head pumps may be reduced by restricting the flow without changing the pump speed, with positive displacement pumps a fixed volume of displacement is delivered for every stroke or revolution; the output can only be changed by altering their internal geometry.
Unlike velocity head pumps, which are usually centrifugal, positive displacement pumps appear in an enormously wide variety of types: gear pumps, pistons, peristaltic, sliding shoes, swash-plates, eccentrics, sliding and rotating vanes, three-, four-or more pointed rotors, helices, progressing cavities, and other types. Some may rely on the lubricity of the fluids pumped and on very small clearance to minimize internal leakage and hence pressure loss.
Positive displacement pumps are sometimes called upon to move fluids that are both corrosive and abrasive. Examples may vary from chocolate, toothpaste, and peanut butter, which require a high level of cleanliness, to coal and tungsten carbide slurries. Conventional pumps handling such products have maintenance costs and repair frequency that might be unacceptably high.
For moving abrasive and corrosive fluids, two special types of positive displacement pumps, have been heretofore available: (1) The Moneaux pump, introduced in the 1930's and sometimes known as the "Moyno" or "progressing cavity" pump, consists of a two plane, single helix, floating rotor, turning inside a double helix stator. The latter is often made of rubber, to improve the resistance to fluids with abrasive entrained solids. The rubber facing also provides a degree of sealing between the rotor and stator. The stator may be made of other materials with special resistance qualities as needed. As better understanding of wear resistance developed, these pumps became available in a variety of different material combinations which increase their useful lives. (2) The Megator type introduced in the 1940's; it is also somewhat tolerant of abrasive fluids; a metal component works with one which is rubber or plastic faced. This positive displacement pump uses a circular eccentric, rotating in the close-fitting cavity in a shoe that is caused to reciprocate against a plate incorporating inlet and discharge ports.
Besides abrasiveness and corrosiveness, a third quality that can impose difficulties is high viscosity, which may vary considerably with temperature. External bypasses can be used to alter the delivery volume without altering the pumps' total throughput, but this does not solve the viscosity problem.
The ability to alter the rate at which the fluid is delivered may be essential or at least desirable for many industrial processes. Special pumps heretofore known which can provide continuous flow adjustment, such as swashplate pumps, are unsuitable if problems such as abrasiveness, corrosiveness and varying viscosity may be present to any substantial extent. Throttling of positive displacement pumps is not possible. By-passing involves mechanical complication and wastes power. Moreover, some pumped fluids are adversely affected by being treated roughly, and may suffer from chemical or structural changes. A practical flow control method is to vary the pump speed, using a multi-speed gearbox or a variable speed motor. These expedients are costly and complex by comparison with a pump whose delivery rate is easily adjustable, as here disclosed.